Dendritic spines are numerous small membrane appendages on the surface of dendrites. Mature dendritic spines, which typically show either a mushroom-like or stub-like shape, develop from thin, filopodia-like protrusions. This morphological maturation of spines occurs during development and the process of learning, while abnormal spines have been demonstrated in patients with mental retardation and autism. We have recently found that the cell surface heparan sulfate proteoglycan syndecan-2 plays a functional role in the development of dendritic spines. Syndecan-2 becomes concentrated in dendritic spines concomitant with their morphological maturation. More importantly, transfection of syndecan-2 induces the formation of mature spines in young neurons which normally do not contain mature spines. Deletion studies have indicated that the C-terminal EFYA motif is required for the spine- inducing activity of syndecan-2, whereas the clustering of syndecan-2, which is a process preceding spine maturation, requires the rest of the syndecan-2 cytoplasmic domain. This project is aimed at elucidating the molecular interactions and signaling events involving the syndecan-2 cytoplasmic domain, leading to improved understanding of the mechanism of dendritic spine development. Toward this goal, we will: 1. Investigate the role of phosphorylation of the syndecan-2 cytoplasmic domain in syndecan-2 clustering in dendritic spines and their morphological maturation. 2. Identify neuronal molecules interacting with the C1 and V regions of the syndecan-2 cytoplasmic domain. 3. Determine the role of Rho family GTPase in syndecan-2 induce spine maturation. 4. Characterize P24syn2, a novel protein that binds to the EFYA tail of syndecan-2, in comparison with other known EFYA-binding proteins, CASK and syntenin, and identify physiological relevant EFYA-binding proteins involved in syndecan-2 induced spine maturation.